1. Field of the Invention
Dihydrocarbyl carbonates are important commercial compounds. The most important of these carbonates, dimethyl carbonate (DMC), for example, is used in enhancing the octane number of gasoline. Since octane enhancement of gasoline represents an immense market, and in view of the need to substitute non-lead containing compounds for this purpose, this compound has grown in commercial importance in recent years. The use of DMC has also been suggested for use as a less toxic and less polluting substitute for phosgene or dimethyl sulfate in reactions which normally require the use of one of these compounds.
A well known method of synthesizing dihydrocarbyl carbonates in general and DMC in particular involves the reaction of an alcohol, methanol in the case of synthesizing DMC, with carbon monoxide and oxygen in a catalytic oxidative carbonylation reaction. A plurality of references have been published directed to the major area of development in this process, the identity of the catalytic agent employed in the oxidative carbonylation reaction.
The processes of the prior art typically utilize a copper-containing catalyst. However, these copper-containing catalysts are characterized by the inclusion of halogen. Halogen is lost from the catalyst during the carbonylation reaction. Thus, replenishment of halogen is required during the reaction. To that end, at least one process of the prior art has suggested the introduction of a halogen source during the reaction. Usually, the halogen source is a hydrogen halide such as hydrogen chloride. These highly acidic agents introduce an important corrosion problem. Indeed, the use of a halogen-containing catalyst introduces corrosion problems necessitating the use of corrosion resistance process equipment, such as reactors, conduits and the like, in these carbonylation processes.
Another problem associated with the use of halogenated, usually chlorinated, catalysts is that in many cases the copper halide is not chemically bound to the catalytic support. As such, the reaction cannot occur in the liquid phase. The use of a liquid alkanol washes the catalytic agent from the catalyst support, increasing the rate of depletion. Thus, reactions utilizing catalysts with unbound copper halides on a support often occur in the gas phase. Those skilled in the art are aware that carbonylation reactions involving methanol are more efficiently run in the liquid phase.
To overcome these detrimental factors, some attempts have been made to utilize copper-containing catalysts which do not include halogen in the oxidative carbonylation of dihydrocarbyl carbonates. However, in these attempts, the catalyst utilized, although overcoming the detrimental feature of halogen containment, have been homogeneous. Those skilled in the art are aware that homogeneous catalysts are very difficult to separate from the product. This difficult separation, which results in not only loss of catalyst but, more importantly, product impurity, emphasizes the non-desirable nature of such homogeneous catalysts.
The above remarks establish the need in the art for a new catalytic process in the oxidative carbonylation of dihydrocarbyl carbonates synthesized from an alcohol, carbon monoxide and oxygen. Such a process should employ a catalyst which overcomes the problems associated with the catalytic agents used in the prior art.
2. Background of the Prior Art
A plurality of references describe processes in which carbon monoxide and oxygen are reacted with monohydric alcohols in general, alkanols in particular and methanol specifically, to produce esters in general, dialkyl carbonates in particular and dimethyl carbonate specifically. These references all involve the use of catalyst systems. These catalyst systems include at least one catalytic agent, oftentimes two or more. The more relevant of this body of prior art is discussed below.
U.S. Pat. No. 3,846,468 to Perrotti et al. is directed to a process in which an alcohol is reacted with oxygen and carbon monoxide in solution in the presence of a cuprous chloride complexed with an organic base bound to the copper atoms by coordinate bonds to produce dihydrocarbyl carbonates. This process is conducted in solution requiring the carbonate product separation from the catalyst by distillation.
U.S. Pat. No. 3,980,690 to Ciprani et al. describes a process related to the '468 patent. The primary distinction between the two processes is that the catalyst system is heterogeneous. The heterogeneous catalyst of the '690 patent consists of a complex of cuprous chloride and poly-4-vinyl pyridine.
Another process for forming carbonic acid esters is disclosed in U.S. Pat. No. 3,952,045 to Gaenzler et al. in which an alcohol is reacted with carbon monoxide and oxygen in the presence of a catalyst comprising copper chloride or copper bromide and an organic phosphorous compound. The copper chloride or copper bromide may be substituted with another copper salt. However, in that case a chloride or bromide of a metal other than copper, soluble in the reaction medium, is present in the reaction medium.
An oxidative carbonylation reaction involving the reaction of an alcohol with a mixture of carbon monoxide and oxygen is disclosed in U.S. Pat. No. 4,076,949 to Zehner. This process is characterized by the presence of a catalytic mixture of (1) a palladium, rhodium, platinum, copper or cadmium metal salt or mixture of such salts; (2) ammonia or an organic amine; (3) a copper or iron salt; and (4) an ammonium salt or an acid with a counterion other than a halide.
The catalytic agent in a reaction to form carbonic acid esters from the reactants, an alcohol, oxygen and carbon monoxide in U.S. Pat. No. 4,218,391 to Romano et al. is a cuprous or cupric salt having a single inorganic anion. Preferably, the copper salt is cuprous bromide, copper perchlorate or cuprous chloride.
The process of U.S. Pat. No. 4,318,862 to Romano et al. involves the formation of dimethyl carbonate by the reaction of methanol, oxygen and carbon monoxide in the presence of a copper salt catalyst. The improved feature of this process involves the utilization of an addition gaseous reactant, hydrogen, which produces hydrogen-enriched synthesis gas as a byproduct.
The carbonylation reaction of U.S. Pat. No. 4,360,477 to Hallgren et al. differs from the usual reaction of an alkanol with carbon monoxide and oxygen in that the reaction involves the recycling of an azeotropic mixture of methanol and dimethyl carbonate which then becomes the reactant. The catalyst used in this carbonylation reaction is cupric chloride or cupric bromide.
U.S. Pat. No. 4,361,519 to Hallgren describes another catalytic process for the preparation of dihydrocarbyl carbonates in which an alcohol is reacted with carbon monoxide and oxygen in the presence of a Group VIIIB noble metal, oxygen, a redox cocatalyst and a Bronsted base, which is an inorganic or organic base.
In an oxidative carbonylation reaction to produce carbonic acid esters from an alcohol, carbon monoxide and oxygen in the liquid phase, the process of U.S. Pat. No. 4,370,275 to Stamman et al. teaches the use of a catalyst system that includes copper and/or copper ions; one or more anions selected from the group consisting of an oxide anion, a hydroxide anion, a carbonate anion and mixtures thereof; halide ions; one or more nitrogen bases; and optionally, one or more ions of a Group II metal, a lanthanide group metal, an actinide group metal and/or a metal of atomic numbers 25 to 30.
The catalyst system of U.S. Pat. No. 4,426,331 to Drent to produce a carbonate ester from alcohol, carbon monoxide and oxygen includes a catalyst system which, in addition to a cuprous compound, comprises a sulfone.
A process for producing dihydrocarbyl carbonates which comprises contacting an alcohol in an oxidative carbonylation reaction with oxygen and carbon monoxide in which no halide is introduced is taught in U.S. Pat. No. 4,604,242 to Harley et al. In this process a homogeneous catalyst system, which comprises bis((2,4-pentanedianato)-copper(II)methoxide) and a basic nitrogen-containing coordination compound, is dissolved in the reaction solution. The carbonate product is thus obtained as part of an azeotrope with the alcoholic reactant.
The process of preparing a dihydrocarbyl carbonate from an alcohol, carbon monoxide and oxygen in U.S. Pat. No. 4,625,044 to Curnutt involves a vapor phase reaction with a catalyst which comprises a nitrogen-containing coordination compound-copper hydrocarbyloxy halide complex supported on activated carbon.
U.S. Pat. Nos. 4,636,576 and 4,638,076, both to Bhattacharya, disclose the preparation of dimethyl carbonate from methanol, carbon monoxide and oxygen in the presence of a catalyst system containing cupric methoxychloride. In the '576 patent the curpic compound of the catalyst system is supplemented with a nitrogen-containing cyclic amide. In the '076 patent the catalyst system includes a phosphoramide in addition to the aforementioned cupric compound.
A process for preparing dihydrocarbyl carbonates, comprising contacting an alkanol, carbon monoxide and oxygen, all in the vapor phase, with a heterogeneous catalyst which comprises a metal halide impregnated on a support, is set forth in PCT Publication WO 87/07601 to Curnutt. The metal of the metal halide is copper, nickel, iron or cobalt. Of these metals copper is preferred, with cupric chloride being the halide of choice. The above metal halide may also be supplemented with an alkali metal or alkaline earth metal chloride. The preferred support is activated carbon.
The above extensive analysis of the prior art establishes the absence therein of a non-halogen-containing heterogeneous copper catalyst. Thus, the problems mentioned above are not completely addressed by the extensive prior art related to processes for forming hydrocarbyl carbonates.